Traditionally, innate immune cells have been considered non-specific and thought to respond identically when re-encountering the same pathogen. In a mouse model of CMV infection, we have shown that NK cells have several features that are normally attributed exclusively to the adaptive immune system. After infection with MCMV NK cells expressing the activating Ly49H receptor, which directly recognizes the MCMV m157 glycoprotein, undergo extensive proliferation. After control of the virus, the population of Ly49H+ NK cells contracts, but generates long-lived memory NK cells capable of enhanced cytokine production and cytolytic activity upon secondary encounter with the pathogen. Additionally, we have shown that co-expression of inhibitory Ly49 receptors reactive with self-MHC class I ligands restrain the response of these Ly49H+ NK cells during viral infection, such that NK cells without a self-MHC class I inhibitory receptor dominate the immune response to MCMV. The evolutionary advantage of maintaining NK cells that lack self-MHC class I reactive inhibitory receptors might be to ensure optimal responses to viral infection, despite the potential risk of attacking healthy cells. The overall goal of this grant is to characterize the nature of memory mouse NK cells using the MCMV model, to determine whether mouse NK cells are capable of generating memory against other viruses, and finally, whether human NK cells specifically recognize human CMV and also possess traits similar to mouse memory NK cells. The specific aims are: 1) To characterize the molecular and functional characteristics of long- lived memory mouse NK cells responding to mouse cytomegalovirus and to establish whether vaccination of NK cells is possible. 2) To determine whether mouse NK cells can specifically respond to viruses other than cytomegalovirus and acquire memory traits. 3) To test the hypothesis that the CD94-NKG2C receptor on NK cells recognizes human cytomegalovirus, and that interaction of this receptor with its ligand leads to generation of memory NK cells.